<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<title>Document</title>
	<style>
		div {
			border: 5px solid red;
			margin-bottom: 20px;
		}

		div.txt {
			background-color: #33FFCC;
		}

		.txt {
			font-weight: 700;
		}
	</style>
</head>
<body>
	<div>元素选择器效果</div>
	<div class="txt">交集选择器效果</div>
	<span class="txt">类选择器效果</span>
</body>
</html>